The aim of the current study was to evaluate the effect of two melatonin concentrationson in vitro maturation and DNA integrity of bovine oocytes by using comet assay. Ovariesfrom cows (4-8 years old) were collected from abattoir in Beni-Suef Province, Egypt andtransported to the laboratory within one hour from the slaughter. Oocytes were recoveredby slicing technique and matured in tissue culture medium 199 (TCM-199) supplementedwith 50 3g / ml gentamycin sulfate, 10% fetal calf serum (FCS), 5 3l / ml L-glutamine, 0.2i.u Follicular Stimulating Hormone (FSH), 2.0 i.u Human Chorionic Gonadotrphin (HCG)and 1.0 3g Estradiole (E2) / ml without or with melatonin (10 or 100 3M / ml). Following 24hours, the maturation rate was determined according to cumulus mass expansion.Moreover, oocyte DNA was evaluated by comet assay. Supplying in vitro maturation
medium with 103M melatonin significantly increased maturation rate as compared withcontrol. On the other side, 100 3M melatonin in maturation medium had no significanteffect on in vitro maturation rate. In addition, results of the comet assay showed that bothmelatonin concentrations had no effect on in vitro matured oocyte DNA as compared tocontrol group. In the current study, it was concluded that supplying in vitro maturation medium with 103M melatonin significantly increased maturation rate while additional increase in melatonin concentration was of no significance. Moreover, melatonin had no effect on in vitro matured oocyte DNA.